(1) Field of the Invention
This invention concerns processes for the preparation of chlorinated phenols. More particularly, it relates to the preparation of para-chloro-phenols by the sulphuryl chloride chlorination of the corresponding para-unsubstituted phenols, and to procedures and techniques for use therein to reduce the likelihood of freeze-up and by-product formation.
(2) Description of Prior Art
p-Chloro-phenols are of value as bactericides and herbicides and as starting materials for making other herbicidally-active materials. They may be prepared by a number of routes. A particularly favoured route is the chlorination of a melt of the phenol using sulphuryl chloride (SO.sub.2 Cl.sub.2) as the chlorinating agent, but there are a number of disadvantages, albeit minor ones, involved in this process. For example, the reaction may easily result in increasing quantities of by-products, such as the o-chloro-phenol and the dichlorinated materials, with less of the desired para compound, particularly if the reaction temperature is high (but at the same time high temperatures are necessary to prevent the reaction mixture solidifying and thus becoming unmanageable). Furthermore, it is highly desirable to recover one of the reaction by-products, SO.sub.2, and convert it back to sulphuryl chloride for re-use, and this recovery may in some circumstances give rise to problems.
The sulphuryl chloride route to the formation of para-chloro-phenols has been known and used for many years, and its disadvantages, and the improvements to overcome them, are well discussed in the literature. For example, in Monsanto's GB Specification No.: 755,676 filed 27/4/54 claiming U.S. Priority of 27/4/53, published 22/8/56, there is disclosed the concept of using a metal halide, specifically aluminum chloride, as a selectivity catalyst in the liquid state sulphuryl chloride chlorination of phenols such as ortho-cresol. There is also described the desirability of selection and control of the temperature throughout the reaction (though there is no real indication of the critical nature of this factor), but the sulphuryl chloride is always added in one portion (there is no disclosure of the employment of separate, discrete amounts of sulphuryl chloride, each addition being separated by a period of time so as to allow the reaction to go to completion).
Coalite's GB Specification No.: 800,630 filed 7/4/54, published 27/8/58, discloses a process for the recovery of the sulphur dioxide generated during the sulphuryl chloride mono-chlorination of aromatic compounds such as ortho-cresol. There are described continuous chlorination processes in which the formed sulphur dioxide is continuously removed by the inventive process; these continuous chlorination processes are all carried out by effectively adding to a melt of the aromatic compound the sulphuryl chloride in a single stage or reaction vessel (there is no reference to adding it in separate, discrete amounts, to a series of vessels), and they are all effected at a constant, and high, temperature (there is no reference to the need for careful temperature selection and control).
In ICI's GB Specification No.: 926,014 filed 3/2/61, published 15/1/63 (equivalent to U.S. Specification No.: 3,318,949), there is disclosed the use of fullers earth, to improve colour and reduce impurities, in the sulphuryl chloride chlorination of ortho-cresol. The Specification clearly appreciates the need for careful selection and control of temperature throughout the reaction (though the advice it gives is contrary to that of Monsanto's GB Specification No.: 755,676 referred to above), but still adds all the sulphuryl chloride in one portion (there is no reference to the desirability of adding the sulphuryl chloride in separate, discrete amounts).
ICI's GB Specification No.: 948,601, filed 16/1/62 published 5/2/64, discloses that the metal halide selectivity-catalysed sulphuryl chloride chlorination of phenols, as described in Monsanto's GB Specification No.: 755,676 referred to above, is much better carried out under anhydrous conditions. It describes how careful reaction temperature selection and control is important, but again adds all the sulphuryl chloride in one portion (there is no reference to any addition in separate, discrete amounts).
In Bayer's German Offenlegungsschrift No.: 1,203,275 filed on 13/12/62, published on 21/10/65, there is described the sulphuryl chloride chlorination of ortho-cresol using as catalyst the metals iron, zinc, tin or aluminium either as metals or as metal alcoholates or phenolates. The Specification describes how the reaction temperature should be selected and controlled during the reaction, but again adds all the sulphuryl chloride in one portion (there is no reference to any addition by way of separate, discrete amounts).
BASF's German Offenlegungsschrift No.: 2,052,821 filed on 28/10/70, published on 4/5/72, describes the sulphuryl chloride chlorination of ortho-cresol in a continuous process using iron pentacarbonyl as catalyst. It describes in some detail the desirability of careful temperature selection and control throughout the reaction, but again effectively adds all the sulphuryl chloride in one portion (it is all metered into the first reactor vessel, and there is no suggestion that it could or should be added in separate amounts--by, say, metering part of it into the second reactor vessel).
Finally, in an early Coalite GB Specification No.: 768,343 (filed 25/6/51 published 13/2/57), there is disclosed the preparation of dichloro-3-methyl-5-ethyl-phenol, for use as a biocide, by the sulphuryl chloride chlorination of a melt of 3-methyl-5-ethyl-phenol isolated from coal tar. In the process described a melt of the 3-methyl-5-ethyl-phenol is first reacted with sufficient sulphuryl chloride to give in a single stage the 4-chloro mono-chlorinated derivative, and in a subsequent step more sulphuryl chloride is added to complete the chlorination to the 2,4- and 4,6-dichloro compounds. Both chlorinations are apparently carried out at a constant temperature between 50.degree. and 65.degree. C. (there is no reference to any need for careful temperature selection and control), and the preliminary preparation of the 4-chloro mono-chlorinated derivative is effected in a single stage (there is no reference to the addition of sulphuryl chloride in discrete amounts separated by a time period allowing the reaction to go to completion).